Method and system for cell equalization with charging sources and shunt regulators

ABSTRACT

A system and method for charging a rechargeable, or secondary, battery including a series string of cells, includes a topology of charging sources that selectively provides charging current to cells that need to be charged, but avoids overcharging cells that are already charged above a predetermined voltage threshold. Based on individual cell voltage measurements, the charging current is controlled in a manner to direct charging current to the battery cell(s) needing charge until these cells are fully charged, and by-passes battery cells that are fully charged or become fully charged.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.60/522,814, filed Nov. 11, 2004, which is hereby incorporated byreference.

FIELD OF INVENTION

The invention generally relates to secondary (rechargeable) batteries,and more particularly, to cell equalization of such batteries.

BACKGROUND OF INVENTION

Generally, secondary (rechargeable) batteries include a string ofindividual battery cells connected in series to obtain a higher outputvoltage level. During charging of secondary batteries, inherentdifferences in the capacity of the individual battery cells may causethe higher capacity cells to achieve full charge first, and thenover-charge while the remaining battery cells continue to charge.Depending on the ability of the cell chemistry to tolerate thisover-charge, cell damage may occur. During discharge, a similar problemmay be encountered when the lower capacity battery cells reach minimumvoltages first and over-discharge. Cell chemistries such as lead-acidand nickel-cadmium may tolerate moderate forms of these conditions,while other cell chemistries, such as silver-zinc and lithium-ion, maybe more easily damaged. The probability of damage due to over-charge maybe further aggravated by demand for rapid charging systems that requirehigher currents and cell temperatures.

For the reasons stated above, charging a series-connected string ofindividual battery cells normally poses unique monitoring and controldifficulties. For example, measuring the voltage of the battery may notnecessarily indicate the condition of each individual cell in thebattery. If the individual battery cells are, for example, not wellbalanced, a cell may experience a damaging over-charge condition eventhough the battery voltage is within acceptable limits. Thus, eachbattery cell in a string usually is monitored and controlled to insurethat each individual cell in the series string does not experience anover-voltage condition during charging.

When charging, secondary battery cells generally are bulk charged if thecell voltage is above a specified level. Bulk charging continues untilany individual cell voltage reaches an upper voltage limit. At the endof bulk charging, one or more battery cells may, however, be onlypartially charged, and may not have yet reached a 100% state of charge.The partially charged condition is considered adequate for someapplications and, thus, the charging process may be terminated prior toeach individual cell being 100% charged. Over time, however, theperformance of individual cells in the battery may diverge due to eachcell being charged to a different level during any one recharge. Tominimize divergence, a second step in the charging process often isimplemented.

The second step in the charging process is known as “cell equalization.”Cell equalization generally begins when a battery cell is clamped at anupper voltage limit during charging. The charging current usuallydecreases because the cell voltage is clamped, and not allowed toincrease. To protect against cell failure, safeguards to terminate thecharging process prior to cell failure often are employed. Cell chargingmay be terminated (and cell equalization ended) based on a specifiedcell charge current level (normal condition), a specified overtemperature condition (fault condition), and/or a specified cell chargetime out (fault condition). At the end of cell equalization, the stringof individual battery cells connected in series generally is consideredat a 100% state of charge even though each individual battery cell maynot be fully charged.

In addition to over charging, battery cells may experience damage if thecell temperature falls outside a specific range. Thus, cell temperaturesare advantageously kept within a specified temperature range during bulkcharging and cell equalization to prevent temperature damage fromoccurring.

Another concern for battery cells is over-discharge. Over-dischargeoften causes serious performance degradation and damage the cell.Over-discharge may occur when any cell voltage drops below a fixedvoltage level. To prevent over-discharge, secondary batteries often areequipped with a mechanism that terminates discharge when any cell dropsbelow a fixed voltage level. Sometimes, however, the cell voltage mayrise after the discharge is terminated, so hysteresis may be necessaryto prevent oscillations.

Thus, it is generally recognized that recharging a secondary batteryhaving a series-connected string of cells preferably is accomplished ina manner that charges each cell to substantially the same level whileavoiding overcharging any of the cells. Thus, there is a need for a cellequalizing charging system that is low-cost, uses simple circuitry,reduces power dissipation during charging, and operates efficiently.

SUMMARY OF INVENTION

A system for charging a secondary battery according to various aspectsof the present invention comprises a plurality of battery cellsconnected in a series string, wherein the series string includes a firstbattery cell at a load end and an nth battery cell at a ground end, anda cell junction located between each respective pair of battery cells.The system, in one embodiment, also includes a plurality of chargingsources, wherein a first charging source is electrically coupled to theload end, and a second charging source is electrically coupled to afirst cell junction between the first battery cell and a second batterycell located adjacent to the first battery cell. In another embodiment,a charging source is electrically coupled to each cell junction formedevery two cells thereafter. In one aspect of an exemplary embodiment ofthe invention, the system includes (n+2)/2 charging sources, while inanother aspect of the invention, there are (n+1)/2 charging sources.

In one exemplary embodiment, the system includes a plurality of shuntregulators, wherein a respective shunt regulator is connected inparallel across each of the second battery cell to the nth battery cell.In one aspect of an exemplary embodiment of the invention, the systemincludes (n−1) shunt regulators connected in parallel across (n−1)battery cells.

In another exemplary embodiment, a charging source is electricallyconnected to each of the plurality of charging sources to providecharging current to each of the plurality of battery cells via theplurality of charging sources included in the system. In a furtherembodiment, the system includes a controller connected to each of theplurality of charging sources, wherein the controller includes circuitryto switch on and off each of the plurality of charging sources. Inaccordance with one aspect of an exemplary embodiment of the invention,the circuitry is configured to allow only one charging source to beswitched on at a time.

In accordance with yet another exemplary embodiment, the system includesa shunt controller coupled to each of the plurality of shunt regulatorsto switch on and off each of the plurality of shunt regulators. Inaccordance with one aspect of an exemplary embodiment of the invention,the shunt controller is configured to switch on a shunt regulator if abattery cell with which the shunt regulator is connected across inparallel is fully charged, and switch off the shunt regulator if thebattery cell is not fully charged. In still another exemplaryembodiment, a controller is connected to each charging source and eachshunt regulator, wherein the controller includes circuitry to switch onand off each of the plurality of charging sources, and switch on and offeach of the plurality of shunt regulators.

Furthermore, in accordance with another embodiment, a plurality of cellmonitoring circuits is included in the system, wherein at least one cellmonitoring circuit is connected to each respective battery cell tomonitor an amount of charge within each respective cell monitor, and incommunication with the controller. In accordance with one aspect of theinvention, the controller switches on only one charging source at atime, and determines if one or more of the plurality of battery cellsneeds to be charged. In accordance with another aspect of the invention,the controller determines a target battery cell, wherein the target cellis at least one of the plurality of battery cells needing to be charged,and is a battery cell located closer to the load end than any other ofthe plurality of battery cells that may need to be charged. Inaccordance with yet another aspect of the invention, the controllerswitches on a target charging source, wherein the target charging sourceis located at a cell junction between the target battery cell and theload end, and the target charging source is located at a cell junctionfarther away from the load end than another charging source locatedbetween the target battery cell and the load end. In accordance withstill another aspect of the invention, the controller switches on andoff each of the plurality of shunt regulators based upon an amount ofcharge within a battery cell associated with each respective shuntregulator.

Various exemplary embodiments of the present invention also include amethod for equalizing voltage of a secondary battery being charged, thebattery comprised of n cells connected in a serial string from a firstcell at one end to an nth cell at another end with a respective celljunction being located between each adjacent cell, the method comprisingthe steps of connecting the plurality of switched charging sources tothe serial string, wherein a first switched charging source iselectrically coupled to the one end of the serial string, and a secondswitched charging source is electrically coupled at the cell junctionbetween the first cell and an adjacent second cell, and a respectiveswitched charging source is electrically coupled at the cell junctionsoccurring every two cells thereafter; connecting a plurality of shuntregulators to the serial string, wherein a respective shunt regulator isconnected in parallel across each of the second cells through the nthcell; and operating the switched charging sources and the shuntregulators to selectively provide charging current to one or more of then cells. In one aspect, the step of connecting a plurality of shuntregulators to the serial string includes the step of connecting (n−1)shunt regulators to the serial string. In another aspect, the step ofconnecting the plurality of switched charging sources to the serialstring includes the step of connecting ((n+2)/2) switched chargingsources to the serial string when n is an even number. In yet anotheraspect, the step of connecting the plurality of switched chargingsources to the serial string includes the step of connecting ((n+1)/2)switched charging sources to the serial string when n is an odd number.

In one exemplary embodiment, the method further comprises the steps ofoperating each of the switched charging sources in one of a first stateand a second state, wherein when a switched charging source is in thefirst state, the source provides a charging current to the respectivecell junction where that switched charging source is electricallyconnected; and when the switched charging source is in the second state,the source does not provide a charging current to the respective celljunction where that switched charging source is electrically connected;and operating each of the shunt regulators in a first state to bypasscharging current around the respective cell across which it is connectedand operates as a high-impedance electrical path in a second state. Inanother exemplary embodiment, the method further comprises the step ofoperating the shunt regulators and the switched charging sources toprovide charging current to each cell having a voltage below apredetermined threshold, and to avoid providing charging current to eachcell having a voltage at or above a predetermined threshold. In yetanother embodiment, the steps of monitoring a respective voltage levelof each of the n cells; and determining which of the n cells is at orabove a predetermined voltage threshold are included in the method.

The invention also includes a second exemplary method for equalizingvoltage of a secondary battery. The second exemplary method includes thesteps of monitoring an amount of charge contained within a plurality ofbattery cells utilizing at least one cell monitor to determine if atleast one battery cell needs charging; transmitting a signal to begincharging operations from the at least one cell monitor when at least oneof the plurality of battery cells needs charging; determining whichcharging source, of a plurality of charging sources, to utilize tocharge said at least one of the plurality of battery cells needingcharging; and switching on an appropriate charging source of theplurality of charging sources to charge said at least one of saidplurality of battery cells needing charging, wherein the appropriatecharging source is determined by its location with respect to at leastone battery cell needing charge. In one exemplary embodiment, the methodincludes switching on at least one shunt regulator coupled in parallelto at least one of the plurality of battery cells, wherein the at leastone shunt regulator is a shunt regulator coupled in parallel across abattery cell including a charge amount greater than a threshold amount.In one aspect of the invention, the second method includes the step ofswitching on at least one shunt regulator occurs prior to said step ofswitching on an appropriate charging source.

In another exemplary embodiment, the second method includes the cellmonitor continuing to monitor the plurality of battery cells until atleast one battery cell receiving charging current is charged to athreshold amount of charge as indicated by the cell voltage, and thecontroller switching on a shunt regulator coupled in parallel to thebattery cell receiving charging current when the battery cell receivingcharging current contains the threshold amount of charge as indicated bythe cell voltage. These steps may be repeated until each battery cellcontains the threshold amount of charge as indicated by the cellvoltage. When each battery cell contains the threshold amount of chargeas indicated by the cell voltage, the charging source and any shuntregulators that were switched on are switched off by the controller.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims when considered inconnection with the drawing Figures, where like reference numbers referto similar elements throughout the Figures, and:

FIG. 1 is a block diagram of one exemplary embodiment of a deviceincluding a secondary battery, and a charging system to recharge thesecondary battery;

FIG. 2 is a block diagram of an exemplary embodiment of a chargingsystem utilizing cell equalization to charge a secondary battery;

FIG. 3 is block diagram of one exemplary embodiment of a topology of thecharging system of FIG. 2;

FIG. 4 is a control truth table and operational chart for the topologyillustrated in FIG. 3; and

FIG. 5 is a flow diagram illustrating an exemplary embodiment of amethod for charging a secondary battery utilizing cell equalization.

DETAILED DESCRIPTION

The detailed description of various exemplary embodiments of theinvention herein makes reference to the accompanying figures anddrawings. While these exemplary embodiments are described in sufficientdetail to enable those skilled in the art to practice the invention, itshould be understood that other embodiments may be realized in thatlogical and mechanical changes may be made without departing from thespirit and scope of the invention. Thus, the detailed description hereinis presented for purposes of illustration only and not by way oflimitation. For example, the steps recited in any of the method orprocess descriptions may be executed in any order and are not limited tothe order presented.

For the sake of brevity, the apparatus and systems (and components ofthe individual operating components) are described in detail herein.Furthermore, the connecting lines shown in the various figures containedherein are intended to represent exemplary functional relationshipsand/or physical couplings between the various elements. It should benoted that many alternative and/or additional functional relationshipsand/or physical connections may be present in a practical system.

Turning now to the figures, FIG. 1 is a block diagram of one exemplaryembodiment of a device 100 including a secondary battery 130 and acharging system 120 to recharge secondary battery 130. Device 100, inone embodiment, includes power source 110. In an exemplary embodiment,power source 110 is a DC power source. In another exemplary embodiment,power source 110 is an AC power source. In one aspect of the invention(when power source 100 is a DC power source), power source 110 may be asolar panel such that power source 100 produces a DC signal. In anotheraspect of the invention (when power source 110 is an AC power source),power source 110 may be a standard AC outlet along with a transformer,or the like, to provide an appropriate voltage signal for chargingsecondary battery 130. The invention contemplates, however, that powersource 110 may be any DC or AC power source known in the art capable ofproviding charging current to recharging secondary battery 130.

Device 100, in another exemplary embodiment, includes charging system120 electrically connected to power source 110. In various aspects ofthe invention, charging system 120 may be suitably configured (asdiscussed in greater detail below) to charge one or more battery cells(not shown) within secondary battery 130.

In one exemplary embodiment, secondary battery 130 is a lithium-ionbattery. In other embodiments of the invention, secondary battery 130may be, but is not limited to, a lead-acid battery, a nickel-cadmiumbattery, a nickel-metal hydride battery, a nickel hydrogen battery, asilver-zinc battery, or any other battery capable of storing a chargeand subsequently being recharged.

Device 100 includes load 140 which, in an exemplary embodiment, is adevice that requires voltage and current. Examples of load 140 include,but certainly are not limited to, a personal digital assistant (PDA), aBlackBerry® device, a cellular phone, a pager, a Palm Pilots device,and/or any other electronic or communication device capable of beingsupplied power by secondary battery 130.

FIG. 2 is a block diagram of an exemplary embodiment of charging system120 of FIG. 1. Charging system 120, in an exemplary embodiment, includescontroller 210, which may be any hardware and/or software suitablyconfigured to switch on and off charging sources 220 and/or shuntregulators 230. As such, controller 210 may be any controller known inthe art capable of switching on and off charging sources and/or shuntregulators when appropriate to do such.

In one exemplary embodiment, controller 210 is connected to at least onecharging source 220 and at least one shunt regulator 230. In otherembodiments, charging system 120 includes a plurality of controllers(not shown) similar to controller 210, wherein a controller is connectedto each charging source 220 to control the operation (i.e., on/offoperation) of their respective charging source 220. In still otherembodiments, charging system 120 includes a plurality of shunt regulatorcontrollers (not shown) similar to controller 210, wherein a shuntregulator controller is connected to each shunt regulator 230 to controlthe operation (i.e., on/off operation) of their respective shuntregulator 230.

The invention contemplates that charging source 220 may be any hardwareand/or software suitably configured to provide charging current to atleast one battery cell if switched on (i.e., operating in a chargingstate), and not provide charging current to a battery cell if switchedoff (i.e., operating in a non-charging state). As such, charging source220 may be any charging source known in the art capable of charging oneor more battery cells. Likewise, shunt regulator 230 may be any hardwareand/or software suitably configured to have a lower resistance than abattery cell connected in parallel if shunt regulator 230 is switchedon, and a greater resistance than the battery cell if shunt regulator230 is switched off. As such, shunt regulator 230 may be any shuntregulator known in the art capable of manipulating the flow of currentinto and/or away from a battery cell connected in parallel to shuntregulator 230.

In another exemplary embodiment, charging system 120 includes seriesstring of battery cells 240 (hereinafter, “series string 240”). Seriesstring 240, in an exemplary embodiment, contains one or more individualbattery cells (not shown), wherein each battery cell voltage isdependent on the cell chemistry. As such, series string 240 may beconfigured to form a secondary battery of any desired voltage.

Charging system 120, in another exemplary embodiment, includes at leastone cell monitor 250 connected to a respective battery cell andcontroller 210. Cell monitor 250 may be any hardware and/or softwaresuitably configured to monitor the terminal voltage of one or morebattery cells. As such, cell monitor 250 may be any cell monitor knownin the art capable of detecting the terminal voltage of an individual orplurality of battery cells. In one aspect of the invention, cell monitor250 may be configured to detect the terminal voltage of a battery cell(with a pre-determined amount of error tolerance). In another aspect ofthe invention, cell monitor 250 may be configured to determine if abattery cell, with which cell monitor 250 is associated, contains aterminal voltage above or below a pre-determined threshold level.Furthermore, cell monitor 250, in an exemplary embodiment, is configuredto communicate the terminal voltage of a battery cell and/or whether thebattery cell contains above or below the threshold amount of charge tocontroller 210. As used herein, the term “above” with reference to aterminal voltage and/or a threshold amount of voltage meanssubstantially the same as or greater than the amount. In addition, theinvention contemplates that charging system 120 may be formed on aprinted circuit board (PCB) (not shown) or on any other platform knownin the art suitable for forming and/or operating charging system 120.

FIG. 3 is a block diagram of one exemplary embodiment of a topology 300of charging system 120. In an exemplary embodiment, topology 300includes a power source (e.g., power source 110) electrically connectedto charging source 305, charging source 310, and charging source 315,wherein charging sources 305, 310, and 315 are each configured similarto charging source 220 discussed above. In one embodiment, chargingsource 305 is connected to and provides charging current to battery cell320 through node 307. Likewise, charging source 310 is connected to andprovides charging current to battery cells 325 and 330 through node 312.Furthermore, charging source 315 is connected to and provides chargingcurrent to battery cell 340 through node 317.

Battery cells 320, 325, 330, and 335, in an exemplary embodiment, arelithium-ion battery cells. In other embodiments, battery cells 320, 325,330, and 335 may be, but are not limited to, lead-acid battery cells,nickel-cadmium battery cells, nickel-metal hydride battery cells, nickelhydrogen battery cells, silver-zinc battery cells, or any other type ofbattery cell capable of storing a charge and subsequently beingrecharged. In addition, the invention contemplates that battery cells320, 325, 330, and 335 may each be any size battery cell known in theart.

Charging sources 305, 310, and 315, in one exemplary embodiment, areeach connected to a controller 370 similar to controller 210 discussedabove. In another exemplary embodiment, controller 370 is also connectedto shunt regulators 350, 355, and 360, wherein shunt regulators 350,355, and 360 are each configured similar to shunt regulator 230discussed above. Controller 370, in one embodiment, is configured totransmit charging source control signals 374 to charging sources 305,310, and 315 to control the on/off operation of charging sources 305,310, and 315. Similarly, controller 370, in another embodiment, isconfigured to transmit shunt regulator control signals 378 to shuntregulators 350, 355, and 360 to control the on/off operation of shuntregulators 350, 355, and 360.

In an exemplary embodiment, shunt regulator 350 is coupled in parallelto battery cell 325 such that shunt regulator 350 is coupled to node 312(i.e., the positive terminal (V+) of battery cell 325) and the negativeterminal (V−) of battery cell 325. Furthermore, shunt regulator 355 isconnected in parallel to battery cell 330 such that shunt regulator 355is connected to V+of battery cell 330, and to node 317 (i.e., V− ofbattery cell of 330). Moreover, shunt regulator 360 is connected inparallel to battery cell 335 such that shunt regulator 360 is connectedto V+ and V− of battery cell 335.

Topology 300, in another exemplary embodiment, includes cell monitor380, cell monitor 385, cell monitor 390, and cell monitor 395, eachbeing configured similar to cell monitor 250 discussed above. In oneembodiment, cell monitors 380, 385, 390, and 395 are connected tobattery cells 320, 325, 330, and 335, respectively, and are eachconnected to controller 370. In an exemplary embodiment, cell monitors380, 385, 390, 395 are each suitably connected to cells 320, 325, 330,and 335 such that cell monitors 380, 385, 390, and 395 are each capableof reading the amount of charge contained within cells 320, 325, 330 and335, respectively. In another exemplary embodiment, cell monitors 380,385, 390, and 395 are suitably connected to controller 370 such thatcell monitors 380, 385, 390, and 395 are capable of communicating theamount of charge (or whether their respective battery cell includescharge above or below the threshold amount) contained within batterycells 320, 325, 330, and 325 to controller 370.

FIG. 4 is a control truth table and operational chart for topology 300,as illustrated in FIG. 3. For the illustrated embodiment of FIG. 3,there are 16 different permutations of the state of charge for batterycells 320, 325, 330, and 335 during a charging operation. Only a fewpermutations will be described in detail herein, as doing so will makethe other states of the control truth table readily apparent. Column 1reflects the 16 different possible permutations of FIG. 3. Columns 2, 3,4, and 5 indicate the state of charge (i.e., fully charged (high) or notfully charged (low)) of battery cells 320, 325, 330, and 335,respectively. Columns 6, 7, and 8 indicate the state of operation (i.e.,on or off) of charging sources 305, 310, and 315, respectively. Columns9, 10, and 11 illustrate the state of operation (i.e., on or off) ofshunt regulators 350, 355, and 360, respectively. Column 12 illustratesthe state of operation of topology 300 (i.e., charging system 120), asillustrated in FIG. 3.

For example, in permutation 5, battery cells 320, 330, and 340 are notfully charged and need to be charged, whereas battery cell 325 is fullycharged and should not be further charged (i.e., over-charged). In thissituation, charging source 305 will be switched on by controller 370since charging source 305 is the charging source electrically closest tobattery cell 325. In other words, the charging source which is: (i)located between a battery cell needing charge that is located closest tothe load, and the load, and (ii) located farther away from the load thanany other charging source(s) that may be located between the batterycell needing charge that is located closest to the load, and the load.Furthermore, shunt regulators 350 and 360 will also be switched on. Assuch, current will flow from charging source 305 and charge cell 320.Also, current will flow through shunt regulator 350 by-passing batterycell 325 since shunt regulator 350 is switched on. Moreover, currentwill flow through and charge battery cells 330 and 335. Thus, batterycells 320, 330, and 335 will receive the necessary charging current, butbattery cell 325 will not receive charging current. Therefore, shuntregulator 350 allows charging current to effectively by-pass a fullycharged battery cell 325 such that battery cell 325 will not becomeover-charged, and possibly damaged.

Permutation 10 is another example of how topology 300 provides chargingcurrent to battery cells needing to be charged, but yet does not providecharging current to cells fully charged. In this example, battery cells325 and 330 need to be charged, whereas battery cells 320 and 325 arefully charged, or are at least contain an amount of charge above athreshold amount. As such, charging source 310 is switched on bycontroller 370 for the same reasons as charging source 305 in the aboveexample. In addition, controller 370 will switch on shunt regulator 360to prevent battery cell 335 from receiving charging current. Hence,charging current is supplied by charging source 310 to battery cells 325and 330, and the charging current flows through shunt regulator 360 toground to avoid overcharging battery cell 335.

Permutation 15 illustrates the example of when only battery cell 335requires recharging. In this example, controller 370 switches oncharging source 315 (for the above reasons) such that charging currentwill flow from charging source 315 through battery cell 335 to ground.As such, battery cells 320, 325, and 330 do not receive charging currentsince they are charged above the minimum threshold amount.

The remaining permutations (i.e., permutations 1-4, 6-9, 11-14, and 16)may analyzed in a manner similar to permutations 5, 10, and 15.Furthermore, the invention contemplates that only one of chargingsources 305, 310, and 315 will be on at any time. As such, the inventionminimizes the amount of charging current that is dissipated during acharging operation.

FIG. 5 is a flow diagram illustrating an exemplary embodiment of amethod 500 for charging a secondary battery utilizing cell equalization.In one exemplary embodiment, method 500 initiates by at least one cellmonitor (e.g., cell monitor 250) beginning to monitor the amount ofcharge in at least one battery cell (e.g., battery cell 320) todetermine if battery cell 320 needs to be charged (step 510). When cellmonitor 250 determines that battery cell 320 needs to be charged, cellmonitor 250, in one embodiment, transmits a signal to a controller(e.g., controller 270) to begin charging operations (step 520). Inanother embodiment, controller 270, if needed, then switches on at leastone shunt regulator (e.g., shunt regulator 230) to divert chargingcurrent from charging any battery cells 320 that are fully charged orcharged above a threshold amount (step 530).

Once any needed shunt regulators 230 are switched on such that chargingcurrent will be diverted around any battery cells 320 not needing to becharged (i.e., to prevent over-charging), in one exemplary embodiment,controller 270 will switch on the appropriate charging source (step540). Which controller 270 switches on is determined in the mannerdiscussed above in the examples discussing permutations 5, 10, and 15 ofFIG. 4. While charging operations are being performed, in oneembodiment, cell batteries 320 are monitored by the cell monitor(s) 250until the threshold voltage is reached in at least one battery cell 320(step 550).

In one embodiment, once the cell monitor(s) 250 transmits a signal tocontroller 270 indicating that at least one battery cell 320 has beencharged to the threshold charge amount, controller 270 switches on theshunt regulator 230 connected in parallel to that particular cellbattery 320 to divert the charging current from further charging thebattery cell 320 (step 560). In another exemplary embodiment, steps 550and 560 may be repeated until each battery cell 320 of series string 240is charged to or above the threshold amount (step 565). After eachbattery cell 320 is charged to or above the threshold amount, controller270 switches off charging source 305 (step 570) and any shunt regulators230 that are switched on (step 580).

Benefits, advantages and solutions to problems have been describedherein with regard to specific embodiments. However, the benefits,advantages, solutions to problems, and any element(s) that may cause anybenefit, advantage, or solution to occur or become more pronounced arenot to be construed as critical, required, or essential features orelements of the invention. All structural, and functional equivalents tothe elements of the above-described exemplary embodiments that are knownto those of ordinary skill in the art are expressly incorporated hereinby reference. As used herein, the terms “comprises,” “comprising,” orany other variation thereof, are intended to cover a non-exclusiveinclusion, such that a process, method, article, or apparatus thatcomprises a list of elements does not include only those elements butmay include other elements not expressly listed or inherent to suchprocess, method, article, or apparatus. Further, no element describedherein is required for the practice of the invention unless expresslydescribed as “essential” or “critical.”

1. A system for charging a secondary battery, comprising: a plurality ofbattery cells connected in a series string, wherein said series stringcomprises: a first battery cell at a load end and an nth battery cell ata ground end, and a cell junction located between each respective pairof battery cells; a plurality of charging sources, wherein: a firstcharging source electrically coupled to said load end, a second chargingsource electrically coupled to a first cell junction between said firstbattery cell and a second battery cell located adjacent to said firstbattery cell, and a charging source electrically coupled to each celljunction occurring every two cells thereafter; and a plurality of shuntregulators, wherein a respective shunt regulator is connected inparallel across each of said second battery cell to said nth batterycell.
 2. The system of claim 1, wherein said plurality of shuntregulators comprises: (n−1) shunt regulators.
 3. The system of claim 1,wherein said plurality of charging sources comprises: (n+2)/2 chargingsources when n is an even number; and (n+1)/2 charging sources when n isan odd number.
 4. The system of claim 1, further comprising: a chargingsource electrically coupled to each of said plurality of chargingsources, wherein said charging source provides charging current to eachof said plurality of battery cells.
 5. The system of claim 1, furthercomprising: a plurality of controllers, wherein a controller is coupledto a respective charging source of said plurality of charging sources,and each of said controllers includes circuitry to at least one ofswitch on and switch off each of said plurality of charging sources. 6.The system of claim 5, wherein said circuitry is configured to allowonly one charging source to be switched on at a time.
 7. The system ofclaim 1, further comprising: a shunt controller coupled to each of saidplurality of shunt regulators, wherein each shunt controller isconfigured to switch on and off each of said plurality of shuntregulators.
 8. The system of claim 7, wherein said shunt controller isconfigured to switch on a shunt regulator if a battery cell with whichsaid shunt regulator is coupled across in parallel is fully charged, andswitch off said shunt regulator if said battery cell is not fullycharged.
 9. The system of claim 1, further comprising: a controllercoupled to each charging source and each shunt regulator, wherein saidcontroller includes circuitry to switch on and switch off each of saidplurality of charging sources, and switch on and switch off each of saidplurality of shunt regulators; and a plurality of cell monitoringcircuits, wherein at least one cell monitoring circuit is coupled toeach respective battery cell to monitor an amount of charge within eachrespective cell monitor, and in communication with said controller,wherein said controller is configured to switch on only one chargingsource at a time, and determines one or more of said plurality ofbattery cells needing to be charged, determines a target battery cell,wherein: said target cell is at least one of said plurality of batterycells needing to be charged, and said target battery cell is locatedcloser to said load end than any other of said plurality of batterycells that may need to be charged, and switches on a target chargingsource, wherein: said target charging source is located at a celljunction between said target battery cell and said load end, and saidtarget charging source is located at a cell junction farther away fromsaid load end than another charging source located between said targetbattery cell and said load end, and wherein said controller switches onand switches off each of said plurality of shunt regulators based uponan amount of charge within a battery cell associated with eachrespective shunt regulator.
 10. A charging system for charging arechargeable battery comprised of n cells connected in a serial stringfrom a first cell at one end to an nth cell at another end with arespective cell junction being located between each adjacent cell, thesystem comprising: a plurality of switched charging sources, selectivelycoupled with a respective one of the cell junctions, wherein a number ofthe switched charging sources is less than (n); and a plurality of shuntregulators, wherein a respective shunt regulator is connected inparallel across each of the second cell through the nth cell, such thata number of the shunt regulators is (n−1).
 11. The charging system ofclaim 10, further comprising: a charging source electrically coupledwith each of the switched charging sources such that the switchedcharging sources may be configured to provide charging current to one ormore of the n cells.
 12. The charging system of claim 10, furthercomprising: a respective cell monitoring circuit configured to measure avoltage of an associated cell; and a controller coupled to each of thecell monitoring circuits, the shunt regulators, and the switchedcharging sources; wherein the shunt regulators and the switched chargingsources are operated by the controller to provide charging current toeach having a voltage below a predetermined threshold and to avoidproviding charging current to each cell having a voltage at or above apredetermined threshold.
 13. The charging system of claim 12, whereinthe controller operates only one of the switched charging sources in thefirst state at any given time.
 14. A method for equalizing voltage of asecondary battery being charged, the battery comprised of n cellsconnected in a serial string from a first cell at a load end to an nthcell at a ground end with a respective cell junction being locatedbetween each adjacent cell, the method comprising the steps of: couplingthe plurality of switched charging sources to the serial string, whereina first switched charging source is electrically coupled to the one endof the serial string, and second switched charging source iselectrically coupled at the cell junction between the first cell and anadjacent second cell; and a respective switched charging source iselectrically coupled at the cell junctions occurring every two cellsthereafter; coupling a plurality of shunt regulators to the serialstring, wherein a respective shunt regulator is connected in parallelacross each of the second cells through the nth cell; and operating theswitched charging sources and the shunt regulators to selectivelyprovide charging current to one or more of the n cells.
 15. The methodof claim 14, wherein said step of connecting a plurality of shuntregulators to the serial string comprises the step of: connecting (n−1)shunt regulators to the serial string.
 16. The method of claim 14,wherein said step of connecting the plurality of switched chargingsources to the serial string comprises the step of: connecting ((n−2)/2)switched charging sources to the serial string when n is an even number.17. The method of claim 14, wherein said step of connecting theplurality of switched charging sources to the serial string comprisesthe step of: connecting ((n+1)/2) switched charging sources to theserial string when n is an odd number.
 18. The method of claim 14,further comprising the steps of: operating each of the switched chargingsources in one of a first state and a second state, wherein when aswitched charging source is in the first state, the source provides acharging current to the respective cell junction where that switchedcharging source is electrically coupled; and when the switched chargingsource is in the second state, the source does not provide a chargingcurrent to the respective cell junction where that switched chargingsource is electrically coupled; and operating each of the shuntregulators in a first state to bypass charging current around therespective cell across which it is connected and operate as ahigh-impedance electrical path in a second state.
 19. The method ofclaim 14, further comprising the step of: operating the shunt regulatorsand the switched charging sources to provide charging current to eachcell having a voltage below a predetermined threshold and to avoidproviding charging current to each cell having a voltage at or above apredetermined threshold.
 20. The method of claim 19, further comprisingthe steps of: monitoring a respective voltage level of each of the ncells; and determining which of the n cells is at or above apredetermined voltage threshold.
 21. A method for equalizing voltage ofa secondary battery comprising the steps of: a) monitoring an amount ofcharge contained within a plurality of battery cells utilizing at leastone cell monitor to determine if at least one battery needs charging; b)transmitting a signal to begin charging operations from said at leastone cell monitor, to a controller, when at least one of said pluralityof battery cells needs charging; c) determining, by said controller,which charging source, of a plurality of charging sources, to utilize tocharge said at least one of said plurality of battery cells needingcharging; and d) switching on, by said controller, an appropriatecharging source of said plurality of charging sources to charge said atleast one of said plurality of battery cells needing charging, whereinsaid appropriate charging source is determined by its location withrespect to at least one battery cell needing charge.
 22. The method ofclaim 21, further comprising the step of: e) switching on at least oneshunt regulator coupled in parallel to at least one of said plurality ofbattery cells, wherein said at least one shunt regulator is a shuntregulator coupled in parallel across a battery cell including a chargeamount greater than a threshold amount.
 23. The method of claim 22,wherein said step of switching on at least one shunt regulator occursprior to said step of switching on an appropriate charging source. 24.The method of claim 21, further comprising the steps of: f) continuingto monitor, by said cell monitor, said plurality of battery cells untilat least one battery cell receiving charging current is charged to athreshold amount of charge; and g) switching on, by said controller, ashunt regulator coupled in parallel to said battery cell receivingcharging current when said battery cell receiving charging currentcontains said threshold amount of charge.
 25. The method of claim 24,further comprising: h) repeating steps f) and g) until each of saidplurality of battery cells contains said threshold amount of charge. 26.The method of claim 25, further comprising the steps of: i) switchingoff, by said controller, said charging source when each of saidplurality of battery cells contains said threshold amount of charge; andj) switching off, by said controller, any shunt regulator switched on bysaid controller.
 27. The method of claim 21, wherein step c) comprisesthe steps of: determining a target battery cell by said controller,wherein: said target cell is at least one of said plurality of batterycells and a battery cell needing charge, and said target battery cell islocated closer to said load end than any other of said plurality ofbattery cells needing charge, and switching on a target charging source,wherein: said target charging source is located at a cell junctionbetween said target battery cell and said load end, and said targetcharging source is located at a cell junction farther away from saidload end than another charging source located between said targetbattery cell and said load end.